In many label applications, it is desirable that the label stock from which the labels are cut be a film of polymeric material rather than paper. Polymeric film can provide properties lacking in paper, such as durability, strength, water resistance, abrasion resistance, gloss, transparency and other properties. Obviously, the polymeric material must meet the practical requirements of commercial manufacture and use. Material costs must be competitive. The film must be formable by a suitable commercial process such as cast film extrusion or blown film extrusion, requiring that the molten film material be flowable to the correct degree to accomplish proper film formation. The formed film must be capable of hot-stretching without deleterious effect, since it is generally advantageous to hot-stretch and anneal the formed film, so as to orient the film and impart a stiffness to the film that is different in machine and cross directions, as described for example in U.S. Pat. No. 5,186,782 and U.S. application Ser. No. 08/055,462 filed Apr. 29, 1993, both of common assignee, the disclosures of which are, to the extent consistent herewith, hereby incorporated by reference as if fully repeated herein. The film must have a printable face and be die-cuttable and matrix-strippable when used in a pressure-sensitive label construction. The labels should have enough "give" or flexibility to conform well to the substrates or containers on which they are used (a particularly demanding requirement when the labels are applied to flexible substrates such as squeezable plastic containers, for example shampoo bottles, but also a requirement with respect to rigid substrates which may have irregularities in their surfaces, such as glass bottles). On the other hand, labels cut from the film should be sufficiently dimensionally stable to maintain print registration and stiff enough to allow them to be properly dispensed as an in-mold label, or to dispense properly past a peel plate or peel-back edge, over which the liner or carrier is stripped, at speeds which are high enough to be commercially viable, i.e. in excess of 200 pieces per minute and preferably in excess of at least about 250 pieces per minute, corresponding to line speeds of about 80 to 100 feet per minute. Dispensing at these speeds generally requires a machine-direction (MD) stiffness of between 10 and 60 Gurley (as measured per TAPPI Gurley stiffness test T543pm). For good conformability, cross-direction (CD) Gurley should not exceed about 0.75 times MD Gurley.
Olefin-SPA blends have been found in many respects to be a preferred material to meet the demands of polymeric die-cut label manufacture. The use of olefin-SPA blends in the manufacture of die-cut labels is shown in aforementioned U.S. Pat. No. 5,186,782 and in U.S. application Ser. Nos. 07/839,369 filed Feb. 21, 1992 now abandoned and 07/942,511 filed Sep. 9, 1992, also of common assignee, the disclosures of which are hereby incorporated by reference as if fully repeated herein. The relatively low cost of the olefinic resins, and their high strength allowing for low caliper film, tend to minimize overall material cost. Hot-stretched polypropylene and/or polyethylene provides stiffness in the machine direction even at relatively low-caliper thicknesses for adequate print registration and dispensing while providing sufficiently low tensile modulus and particularly sufficiently high elongation in the cross direction to allow good conformability. Soft polar additives such as EVA improve printability, and also contribute to conformability since they tend to be soft rubbery materials.
Olefin-SPA blends have been found useful to produce, on an economical basis, printable 3.5 mil coextruded labels die-cut from uniaxially hot-stretched film that are durable, strong and water-resistant, and that generally have good structural and surface properties and performance characteristics, and such labels have been manufactured and sold prior to the present invention. The inclusion of the soft polar additive contributes importantly to label printability, conformability and die-cuttability without too much degrading the stiffness and tensile strength of the final uniaxially oriented product. Uniaxial stretching also makes an important contribution to performance by creating substantial differentials between machine direction (MD) and cross direction (CD) stiffnesses, tensile moduli, elongations, and other properties.
However, poor abrasion resistance has been a problem for such uniaxially oriented olefin-SPA blend films of the prior art. While improving the abrasion resistance of such films promised to expand their markets, the use of strengthening agents to possibly improve abrasion resistance appeared to be ruled out because, in strengthening the film, such agents would increase stiffness and tensile modulus in both MD and CD directions, and in particular would be expected to destroy conformability of the film.
Therefore, strengthening agents such as "compatibilizers" have not been used for uniaxially oriented olefin-SPA blend films, even though use of compatibilizers has long been known for strengthening various resinous film-forming materials comprising incompatible or immiscible blends of polar additives and base materials. Examples of the use of compatibilizers to couple two immiscible film phases together and thereby increase the strength of the film are found in U.S. Pat. Nos. 4,410,482; 4,615,941, 4,770,837 and 5,151,309. As just indicated, to the extent that similar use of compatibilizer in uniaxially oriented olefin-SPA blend films might favorably change some physical properties, properties such as cross-direction elongation would be expected to change in an unfavorable direction in rough proportion to the degree of favorable change of other properties, so that what would be gained, for example, in stiffness would be lost in conformability.